1. Field of the Invention
This invention relates to a method of locating a coil, more particularly to a method of locating a coil which is characterized with a source coil detecting section incorporating a plurality of single axis coils.
2. Related Art Statement
Recently, endoscopes come to be widely used in the medical field and engineering. The endoscope, especially one with an elastic insert can be inserted into a bodily cavity with a sinuous contour, and allows organs in deeper parts of the body to be observed directly without requiring surgical intervention. The endoscope further allows, when necessary, the surgeon to remove polyp or the like with a minute treatment tool inserted through it.
To insert an endoscope through a winding bodily cavity, however, requires more or less skill from the operator, as is obvious from the fact that smooth insertion of an endoscope through the anus to inspect lower digestive tracts would be impossible without a considerable skill.
For an endoscope to make a smooth insertion, it is necessary to flex the curved part of insert in accordance with bends of the body tract it follows while the insert is advancing into the body. Then, it would be valuable for the accomplishment of such feat if it becomes possible to locate the tip of insert with respect to the bodily cavity, or to see how the curved part is flexed at a time under study.
To attain this object, the prior art disclosed in PCT WO94/04938 official gazette introduces a method to locate a coil: three coils are placed at specified positions relative to each other so as to cross their long axes at right angles and to represent three axes of a coordinate system, they generate one after another AC magnetic fields whose vectors intersect at right angles in a space defined by the coordinate system, a separate single axis coil is placed at one position within the same space, and an induced voltage across both ends of the same coil is measured each time the axial coils are excited to generate a magnetic field, and the measured data are utilized to locate the solitary axis coil in the coordinate system.
The method introduced by the above-described prior art disclosed in PCT WO94/04938, however, presents with a problem: a radiofrequency signal to excite the solitary axial coil to generate a magnetic field in it comes to shift from the frequency with which a frequency component extracting means deal, because the oscillating mechanism of field generating coils is sensitive to changes in environment including ambient temperature, or being more or less ready to develop changes after long use; and this discrepancy in frequency will result in a wrong estimation of the location of the solitary coil, or the tip of an endoscope when the coil is incorporated in an endoscope, and thus will make it impossible to make a precise estimation of how the endoscope is inserted into the body at a time of interest.
To meet that problem, the prior art disclosed in Japanese Unexamined Patent Publication No. 9-28661 proposes an endoscope shape detecting apparatus wherein a frequency adjustment means is introduced that can adjust the radiofrequency to coincide with a reference frequency. Accordingly, even when this apparatus is exposed to environment which may invoke changes in the radiofrequency, or has been used over a time long enough to cause changes in the radiofrequency, the coincidence remains uninfluenced from such environmental changes and long use. This apparatus ensures a stable monitoring of the tip of an endoscope.
The method introduced by the above-described prior art disclosed in PCT WO94/04938 has another problem: it requires a plurality of three axis coils each of which comprises a combination of three single core coils crossed at right angles, to give an estimation of the location of a magnetism generating element from the results of the detection elements, and thus the system incorporating those coils is complicated in structure.
The method disclosed in Japanese Unexamined Patent Publication No. 9-28661, when applied to an endoscope, presents the same problem: it requires a plurality of three axis coils each of which is produced by combining three single core coils crossed at right angles, to locate a magnetism generating element based on the measurements by the plurality of detection elements, and thus the system incorporating the coils is complicated in structure.
The method disclosed in Japanese Unexamined Patent Publication No. 9-28661 presents still another problem: in vector analysis, frequency components comprising a signal train receive Fourier transformation, but the thus derived result does not always represent original observed frequencies. Namely, there are more or less leaks. To reduce such leaks it is necessary to introduce a widow function method for the analysis.